1. Technical Field
The invention concerns a spraying device for use in high capacity fluid bed granulation and a method for preparation of solid granules from a liquid material in a fluid bed.
More specifically, this invention relates to spray nozzles usable for supplying granule-growth liquid, by atomizing the liquid in a fluidized bed of solid particles where upon the microscopic liquid drops are sprayed.
2. Description of the Related Art
The fluidized bed granulation process originally designed by Nederlandse Stikstof Maatschappiji, later known as the NSM and now the HFT (Hydro Fertilizer Technology) granulator, has proved to have great advantages over other processes for granulating fertilizer substances like urea and ammonium nitrate, see for example U.S. Pat. No. 5,653,781. The HFT granulation process combines very high capacity with excellent product properties and low energy consumption. In achieving this, the way of spraying the liquid to be granulated into the fluidized bed is of the highest importance. The design of the nozzles where the liquid is atomized influence the liquid flow capacity—and thus the capacity of the granulator—as well as the pressure and volume flow of atomization gas required to maintain spouts with continuous and uniform sprays without agglomeration and unwanted disturbance of the fluidized bed. Finally the nozzle design also influences the energy consumption of the granulator through its flow and pressure characteristics.
State-of-the-art spraying devices (nozzles) are generally of the binary type atomizing nozzles, where a gas, e.g. air, is used to atomize the liquid to be granulated. In principle the gas and liquid may be introduced through the nozzle in the same single tube, or gas and liquid may be introduced separately through for example concentric tubes so that the mixing takes place only in and after the nozzle aperture. The principle aims are effective gas-liquid mixing; uniform and small size of liquid particles and a suitable geometry of the resulting spray or spout, so that the atomized liquid (microscopic droplets) is continuously and homogenously accreted on the granules formed in the fluidized bed.
Most nozzle designs available are not designed for use in high capacity granulation. Nozzles for granulation must be able to handle concentrated solutions (up to 99%) or melts without clogging or other problems ruining the production process. At the same time, there are notable differences between different designs with regard to energy consumption of the process, capacity (product throughput) and quality of the formed granules. Existing and previous nozzle types need relatively high levels of atomization gas, at elevated pressures, which again means high energy consumption.
Nozzles for use in a granulation process are for example described in U.S. Pat. No. 4,701,353. These have a central channel through which the liquid material is supplied, and a channel concentric therewith carrying a powerful gas stream. The liquid is passed through a rotation chamber before it is mixed with the gas stream. The nozzle may also have a concentric outer channel for providing a less energy-rich gas stream. However, this nozzle design has limitations in liquid flow capacity and in the energy required for both nebulizing or atomizing the liquid into suitable droplets and at the same time fluidizing the droplets into a properly shaped spout where the particle growth will occur. The rotation chamber described in U.S. Pat. No. 4,701,353 will only serve to give the liquid a helical movement, while the mixing chamber of the novel device described below will allow a thorough mixing of the atomizing gas and liquid before the spray leaves the exit aperture of the nozzle. Thereby the kinetic energy from the gas and liquid is utilized optimally. The novel device and method described herein demonstrate high granulation capacity and low energy consumption; several times better than the values quoted in U.S. Pat. No. 4,701,353. In cases where a second concentric gas tube is fitted on the nozzle, like described in claim 5 of U.S. Pat. No. 4,701,353, such design complicates the device unnecessarily compared to the device described below, since the novel device and method makes use of a portion of the fluidization gas supply for the extra spout gas channelled up around the nozzle.
International patent application WO 02083320 also describes a nozzle for use in fluid bed granulators. It has a central supply for liquid where the liquid thereafter is led through a swirling device and into an internal mixing chamber. Gas is supplied to the mixing chamber through a plurality of openings in the wall in the lower part of the mixing chamber. However, the description reveals no examples and no figures relating to the practical applicability of the described design, whereas the novel device and method described below has proved itself through high production capacity, good product quality properties and low energy consumption, in comparison with any other previously described designs.
The object of the invention is to obtain a granulation process with reduced energy consumption and enhanced granulation capacity; both factors reducing the variable costs of the granulation step. Another object is to obtain granules of better quality. These objects of the invention are obtained with the device and method as described below, and the invention is further defined and characterized by the accompanying patent claims.